Aircraft: C172
Phase of flight: Landing
Anomaly: Ground Event / Encounter Object; Inflight Event / Encounter Unstabilized Approach

I was training a student who has approximately 30 hours and who has soloed 3 times. We were doing a training flight in the traffic pattern working on short and soft field takeoffs and landings. For the short field landings; I was giving the scenario that a previous student of mine had received during his private pilot checkride. Simply enough; the examiner wanted him to land on the threshold. So that is the same scenario that my student and I were practicing. We had performed 9 takeoffs and landings; and he was doing very well with the spot landings and short field procedure. In the beginning I was following very closely on the controls to ensure the proper threshold crossing height for the point which we were landing. As the lesson progressed; I slowly eased off of the controls to allow him to be more in control. After the 9th landing; I knew it was about time to finish up for the evening. My student asked if we could do just one more takeoff and landing; to which I agreed. The sun had set and we were beginning to lose some of our light. The lesson before this was his first night flight where we did 7 full stop takeoff and landings; so I wasn't terribly concerned with a little bit of light loss. While we were on the downwind leg; I told my student that we would try to touchdown at the beginning of the runway; just before the numbers one last time. As we turned onto final; the lighting system was not turned on. I preferred it that way; but failed to vocalize to my student to keep them off. When we approached short final I heard him keying on the lights. Before I could say anything; the lights were coming on. He had turned them on high intensity. As we kept coming down; I began reaching for the hand held mic to turn down the lights. We were low enough; I thought the best decision was for me to try to get them and let him focus on the landing. When I got the mic and got them keyed down; we were probably 20 ft above the ground. I made a quick glance over to his airspeed indicator to verify he was at the proper speed and to put the mic back so my hands were free. When I looked back; we were beginning to cross the threshold and I knew we had gotten a little lower than I would have liked and we then felt the RH (right hand) tire hit the threshold light. It didn't alter the course of the airplane; and we were able to touchdown straight; on the main wheels; and in the center of the runway. We didn't actually realize it was a light until we taxied back to the beginning of the runway to make sure it wasn't a light. Sure enough; there it was. It knocked it off its base and broke the glass. We taxied in and shutdown. We did a visual inspection of the entire aircraft and there was no evidence of any damage. I immediately called the airport foreman to report what had happened so he could get a hold of the proper maintenance technician.Looking back; there are a few things I; as the instructor; could have done differently to prevent this situation. I should have stuck with my instinct that we had done enough takeoffs and landings and that any more could be detrimental to the progress made. Next; I could have made the decision to pick a point further down the runway due to the diminishing sunlight. Third; when my student keyed the lights to high intensity; I should have had him adjust to land farther down the runway due to the effect it was having on our vision. I have learned a lot from this situation and will never let it happen again. I am trying my hardest to do everything properly to fix the situation. C172 flight instructor reported striking a runway threshold light during landing with his student at the controls.